Manufacture of thorium



July 27, 1948. H. c. RENTSCHLER ET AL. 2,446,062

MANUFACTURE OF' THORIUM 2 Sheets-Sheet l Filed Jan. 23, 1945 @@@Emw Gi' TEHP )VHA/0144575' Juuy 27, 1948 H. c. RENTscl-ILER ET AL 2,446,062

MANUFACTURE OF THORIUM 2 Sheets-Sheet 2 Filed Jan. 23,V 1945 /f YPZ/YUL IC PRESS' Patented July 27, 1948 UNITED STATES PATENT QFFICE MANUFACTURE F THORIUM Application January 23, 1945, Serial No. 574,130

(Cl. 'l5-84) 9 Claims. l

This invention relates to thorium, more particularly to the production of meta-l of an exceptionally high degree of purity, and to an improved method for the manufacture thereof.

The principal object of our invention, generally considered, is to produce thorium by reaction of the oxide with calcium, said reaction taking place in an open cup enclosed in a container, said container being lled with an inert gas, as distinguished from the prior practice of reducing the oxide in a heavy-walled iron bomb with a ground-in stopper, Said bomb being heated in open air.

Another object of our invention is to produce thorium powder by heating the oxide and calcium by high-frequency induction, the reaction cup being enclosed in a high-silica glass bell jar, such as one of 96% silica glass.

A further object of our invention is to treat the powder produced in accordance with the above, to consolidate it into coherent metal and to form to the desired shape.

Other objects and advantages of the invention will become apparent as the description proceeds.

Referring to the drawing:

Fig. 1 is a diagrammatic view, with parts in section, of apparatus for producing thorium in powder form.

Fig. 2 is a vertical sectional view of apparatus for leaching the contents of the cup used in the reduction of thorium oxide, to remove the thorium produced therein.

Fig. 3 is a vertical sectional view of appara- 'tus for washing the thorium powder produced.

Fig. 4 is an elevational view of apparatus, with parts in vertical section, illustrating how the powder produced may be washed and dried.

Fig. 5 is a vertical sectional view, with parts in elevation, of apparatus for completing the drying of the thorium powder.

Fig. 6 is a vertical sectional view', with a part in elevation, of apparatus for producing slugs from thorium powder.

Fig. 7 is a vertical sectional view of apparatus for melting a slug' of thorium, such as produced by apparatus shown in preceding figures. Fig. 8 is" a vertical sectional View of a hydraulic press` which maybe employed for reshaping the pellets of thorium produced by melting, prior to iirial forming.

Fig. 9 is a vertical sectional View of rolls which may be employed for forming the' produced thorium intoslreet.V l lo is avertical sectional View of a die which may be used for drawing thorium into wire.

The reduction of rare metal oxides, including thoria, by calcium or other reducing agents, has previously been accomplished in heavy-walled iron bombs, with a ground-in stopper held in place by a screw cap. Such devices have a number of limitations and disadvantages, to wit:

It is difficult to maintain Ian air-tight joint between the stopper and bomb and toprevent reoiL'dation of the thorium powder produced as the bomb cools, or during the heating process. Considerable warping of the bomb occurs during heating and cooling,v thus necessitating time-consuming lapping operations between runs. The construction of bombs has been limited to materials which will resist oxidation at elevated temperatures, and iron or iron alloys have been generally used for economy. Bombs were without exception of heavy-walled construction to permit suflici'ent surface area for sealing and presumably to withstand pressure produced in the reaction.

From thermo-chemical data and a consideration of the products formed in the reaction'y b'etween thorium oxide and calcium, 'we concluded that the pressures ydeveloped in the reaction were insufficient to necessitate the heavy-walled bombs previously used. We tested our conclusions by placing an open iron cup under a highsilica glass bell jar, evacuating the bell jar, and then heating the iron cup by high frequency induction to cause the calcium to reduce the thorium oxide. Vaporization of the calcium was suppressed .by lling the jar with argon gas at a pressure slightly less than atmospheric. There was no abnormal pressure produced during the reaction, very little vaporization of calcium, and satisfactory thorium metal was obtained. u

The invention to be disclosed, therefore, differs from prior practice in the following respects and obviates several limitations of former practice. The heating, reaction, and cooling of the charge is carried out under known, controlled, and reproducible conditions of gas iilling and exhaust. The reaction is carried out in an o'pen vessel or cup of relatively thin` wall. Oxidation of the cup and thorium produced is entirely prevented` by working in an atmosphere free of oxygen. A choice of several materials of construction for the cup may be made, the only limitation being the melting point and chemical activity of the material with thorium and calcium at elevated temperatures. Thus the cup may be made out of a metal of4 high melting point,

such as molybdenum, which permits the reduction to be made at temperatures at least up to 1400" C. or higher. The use of such temperatures results in an increase in the fluidity of the charge and a faster and more complete reduction.

In Fig. 1- there is shown apparatus for eiecting thereduction of thorium oxide to metal. This involves a cup or crucible I I formed of a suitable metal, relatively inert to the charge and thorium at elevated temperatures, preferably molybdenum, although a molybdenum-lined iron cup, or one formed of iron or steel may be employed. However, if thorium of a very low iron content is desired, molybdenum or its equivalent must be used. The cup is supported above a metal plate I2 which is preferably cooled by circulating water therethrough by means of inlet pipe I3 and outlet pipe I4. The cup preferably rests on a hollow refractory insulator I5. thoriumoxide and granulated, distilled calcium (5 to 10 mesh) is placed in the cup. The preferred mixture is 400 gms. of thorium oxide and 240 gms. of calcium, which represents an excess of vapproximately 98% or 100% over that theoretically required, in accordance with the equation:

AThis proportion is preferred, although satisfactory metal has been produced with as low as 75% excess of calcium. As previously mentioned, it is desirable to produce as fluid a condition of the charge as possible during the reduction, and increasing the excess of calcium promotes this conhigh vacuum pump, or mercury diffusion pump,

and a liquid air trap I'l. A 96% silica bell jar I8, just large enough to slip over the cup, is set on the metal base and sealed vacuumtight, preferably by means of vacuum wax I9, to provide a gas-tight enclosure. to a high vacuum through valve or stop cock 2l, a Geissler tube 24 serving to indicate the degree 'of exhaust obtained.

A high frequency coil 23 is then placed around ,the bell jar I8 and the cup slowly heated by induction to approximately 700 C., without substantially heating the enclosure and thereby maintaining the gas-tight condition, pumping out any gas evolved and continuing this operation until the degree of exhaust is below that supporting a discharge in the Geissler tube 24. Argon gas (99.7 is then introduced, as from tank through valve 2I to a pressure of about twothirds to one atmosphere. A mercury column 22 indicates this pressure. A gas trap or blow-oit '26, cornprising a mercury column, is also provided. The metal cup II is then slowly heated by energizing the coil 23 to melt the calcium and reduce the thorium oxide to metal powder.

Under the ,conditions specified, the reaction apn pears to start at a temperature of approximately 800 C. to 900 C., noted by the appearance of a ne white mist, which is calcium vapor. When this is observed, the temperature is raised to approximately 1400 C. and the cup held at this temperature` for a period of about ten minutes. This moderate period of heating has been found to be sufficient for a satisfactory reduction at the temperature stated.

After the cup land charge have thoroughly A mixture of n The jar is then exhausted cooled, the cup is removed and placed in a jar or receptacle 21 containing water and acetic acid, which dissolves the excess calcium and the calcium oxide formed in the reaction, as represented in Fig. 2. While in the receptacle 21, the cup II is preferably centered, asby means of a block of wood 28, and cooled during the process of leaching as by means of a coil of pipe 29, preferably stainless steel, through which water circulates. `The mixture of water and acetic acid is preferably stirred as by means of a stainless steel motor-driven stirrer 3|.

A desirable manner of leaching is to add 6 to 8 liters of water to the jar 21 of 10 to 12 liters capacity. Acetic acid is added gradually, to

l200 cc. at a time, until approximately one liter has been added, stirring continuously to complete removal of the charge from the cup. This operation usually takes from 3 to 8 hours, depending upon local conditions of heating during reduction.

After the charge is leached out of the cup, the wooden block, molybdenum cup, and cooling coil, are removed, and the powder allowed to settle for 'about 20 minutes. The supernatant liquid is poured ofl and about 3 liters of Water added, plus an equal volume of glacial acetic acid, and stirring is continued for about 1 hour with the propeller 32 of the stirring apparatus well down into the jar to insure maximum agitation, as represented in Fig. 3. The powder is then allowed to settle for about 15 minutes and the supernatant liquid poured oif.

The powder in the bottom of the container is then Washed with water, using 4 to 5 liters per wash, stirring for 5 minutes, and settling for 15 minutes between pouroifs. This operation is repeated 9 or 10 times.

During the first Washings, the supernatant liquid is cloudy due to the presence of fines. These `are not lterable and are discarded. As the supernatant liquid becomes clearer, a white turbidity. appears which is probably due to the reaction of the acid and water on very iine particles. With continued washing, this cloudiness disappears, and when this point is reached the powder is thrown on Bchner funnel 33, as illustrated in Fig. 4, using suction as applied to tube 34 of flask 35. A relatively coarse lter paper is preferably used in the funnel, the tubulation 36 of which passes through .a cork 31 in the neck of flask 35. The powder is washed with water until the filtrate is clear, and then with alcohol and ether in succession.

The metal powder, after removal from the Bchner funnel, is dried in a spherical flask 38 having a neck 39 receiving a cork 4I through which tubes 42 and 43 pass, one of said tubes being connected to a vacuum pump as illustrated in Fig. 5. Such an arrangement permits the powder to be dried under vacuum conditions. The complete removal of moisture is obtained by immersing the flask with the vacuum on in water 44 at 60 C. to '70 C. and shaking the flask intermittently until such removal is noted by the absence of dusting. upon shaking.

Thorium powder, such as produced in accordance with the aforedescribed method, may be worked as by first pressing into pellets and heat treating in vacuo at 1200 C. to 1400 C. After such treatment the metal may be cold deformed by the usual operations such as rolling, swaging, or drawing. Figure 6 illustrates a press 45 in which a quantity of thorium powder may be pressed into a pellet 46, as by a plunger 41, said powder resting on a plate 4B fitting the lower end of said .press 45. Pressures of `about tons per square inch are desirably applied.

`Pressed pellets may be transferred to a crucible or cup 49 for the purpose of sintering or melt- '.ing in vacuo. The only satisfactory refractory `that we have found for melting is beryllia. In practice we have used crucibles 11/2" at the top, l" deep, with the bottom hemispherical. For protection in the event of its cracking, such a crucible is desirably set into an Alundu-m ignition capsule 5i, 1%" diameter by 3A deep.

In order to obtain the necessary close coupling for heating by high-frequency induction, a 96% silica bottle 52 desirably 21/2" outside diameter is used, with an appropriate high-frequency coil 53 f which ts over the bottle with about 1/4 clearance. 4lluring the melting operation, the capsule 54 supported crucible 49, and pressed pellet '46, preferably rest on an insulating refractory block 54 supported by arbo-110W refractory insulator 50 above a metal plate 12a, corresponding or identical with the plate I2 of Fig. 1, cooled and tubulated as in said first ligure. The apparatus for melting may, therefore, be substantially identical with that used for reduction except that the argon -line may be omitted.

With the oscillator available, we have been able `to melt 300` gm's. of thorium in one operation.

This amount could be increased by insulation of the Crucible containing the charge, or by increasing the size of the oscillator. During the melting operation, the .pressed pellet 46 is placed in the 'crucible 49, as viewed in '7, and after the chamber 52 has been evacuated to 10 microns or less, the oscillator power is turned on through icoi-l 53. If the powder is low in oxide content, the heating lshould take place instantaneously. Gas is evolved in the early stages of heating and this is :pumped out continuously, shutting oi the oscillator if the pressure exceeds vapproximately L` microns.

, When the metal becomes molten it is held in this condition 'until quiescent and completely liquid. The oscillator coilis then slowly raised with the current on, and the melt allowed 'to cool. The `coil is raised slowly -to reduce the tendency for fblow holes tto form in the melt during cooling.

Sonie -ide'a of the character of the metal may be obtained by observation of the condition of the melt. The molten metal should appear clear and l relatively lfree fromopaque particles. If the melt appears cloudy and viscous,ip'robably due to oxide inclusions, the metal may not be satisfactory for cold working and will -almost certainly show a tendency to edge crack. A convenient charge for the crucible and coil, as above described, is from 100 to 150 gms. Metal melted in a beryllia crucible contains traces of beryllia.

Fused buttons 8|] of metal produced from Ypressed Ipellets 46 are partiallyspherical inshape because of the shape of the crucible. In Aorder to yconvert these into a Iform suitable for lrolling -or swaging into wire, we have found that they can be reshaped under hydraulic pressure. To per- Lform this operation, the buttons `are placed in a 'i 6 have been slntered using induction heating and lsimilar vacuum techniques, as described under melting.

Thorium metal produced in accordance with our invention analyzes, as fol-lows:

Thorium metal-about 99.5% to about 99.8% Calcium .005% Molybdenum-about .005%

lIronabout .005%

Silica-about .01%

The remainder consists of thoria and spectroscopic traces of common metals including nickel, copper, and aluminum, and beryllia if melted in a Crucible thereof.

Fig. 9 illustrates a pair of rolls 51 and 58 between Which a plate 59 of thorium is being passed [in order to reduce the thickness thereof, as in rolling plates of that metal.

Fig. 10 illustrates a die 6i' through which a rod or wire of thorium is being drawn to the desired size.

From the foregoing it will be seen that we have devised an improved method of manufacturing thorium in which the 'reaction is carried out in a transparent bulb so that the progress thereof can be visually ascertained. By virtue of carrying out the reaction in a closed container and in an inert gas atmosphere, the product is protected from deterioration as the `inert gas vcan neither escape during vthe reaction, nor air or other gas enter the container during cooling.

Although a preferred embodiment of our invention has been described, it vwill be understood that modifications may be made within the spirit and scope of the 'appended claims,

We claim:

1. The method vof manufacturing thorium comprising mixing the oxide thereof with calcium,

. placing vthe mixture in `a molybdenum cup, en-

closing said Acupin a high-silica glass bulb, evacuating said bulb, introducing inert gas thereinto, heating `the cup and contents to between about @800 `and 900 C. to melt 'the calcium and cause it to react with the 4oxide and liberate free thori- "um, cooling the mixture, leaching. and drying the powdered metal produced.

2. The method of manufacturing thorium comprising mixing the oxide thereof with calcium, placing the mixture in a molybdenum cup, Aen- `='closing said cup `in a high-silica glass bulb, evac- "an exceptionally high degree of purity, comprising mixing the oxide thereof with about -excess of granulated distilled calcium, placing the mixture in an open cup of high-melting point met-al relatively inert to the charge and `thorium at elevated temperatures, supporting said cup and surrounding it with a .gas-tight enclosure to protect it and the thorium when produced from oxidation and 'make possible the employment of unusually high reacting temperatures, exhausting said enclosure to a high degree of vacuum, slowly heating said cup to about 700 C. by high-frequency induction to avoid substantially heating the enclosure, while pumping out the gas evolved, continuing the pumping operation to restore `said high degree of vacuum, admitting 't'o said enclosure, to a pressure slightly less than atmos- 'pherid a gas inertto the cup audits contents, then slowly again heating said cup by high-frequency induction to melt the calcium, starting a reaction, as noted by the appearance of a fine white mist, thus reducing the oxide to thorium powder, then by high-frequency induction increasing the temperature of said cup to about l400 C. to increase the fluidity of the charge and quickly effect a reaction more complete than usual and holding it at said temperature for a moderate length of time, cooling and removing said cup from said enclosure, Washing the admixed material from the thorium powder produced, and drying said powder.

4. The method of manufacturing thorium of an exceptionally high degree of purity, comprising mixing the oxide thereof with about 100% excess of granulated distilled calcium, placing the `mixture in a cup of molybdenum, supporting said cup and placing it in a gas-tight enclosure to protect it and. the thorium when produced from oxidation and make possible the employment of unusually high reacting temperatures, exhausting said enclosure to ahigh degree of vacuum, slowly heating said cup to about 700 C. by high-frequency induction to avoid substantially heating the enclosure, while pumping out the gas evolved, continuing the pumpingoperation to restore said high degree of vacuum, admitting to said enclosure, to a pressure slightly less than atmospheric, a gas inert to the cup and its contents, then slowly again heating said cup by high-frequency `induction to melt the calcium and reduce the `oxide to thorium powder, then by high-frequency induction increasing the temperature of said cup to about 1400 C. and holding it at said temperature to effect a reaction more complete than usual, cooling and removing said cup from said enclosure, washing the admixed material from the thorium powder produced, and drying said powder.

The method of manufacturing thorium comprising mixing the oxide thereof with calcium, placing the mixture in a cup of high-melting point material relatively inert to the charge and thorium at elevated temperatures, supporting said cup and placing it in a gas-tight enclosure to protect it and the thorium when produced from oxidation, exhausting said enclosure to a high degree of vacuum, slowly heating said cup to about 700 C., introducing inert gas into said enclosure, slowly again heating said cup to melt the calcium, starting a reaction and reducing the oxide to thorium powder, cooling and removing said cup from said enclosure, washing the admixed material from the thorium powder produced, and drying said powder.

6. The method of manufacturing thorium comprising mixing the oxide thereof with calcium, placing the mixture in a cup of high-melting point metal relatively inert to the chart and thorium at elevated temperatures, supporting said cup and placing it in a gas-tight enclosure to protect it and the thorium when produced from oxidation, exhausting said enclosure to a high degree of vacuum, slowly heating said cup to about 700 C. while pumping out the gas evolved, continuing the pumping operation to restore said high degree of vacuum, admitting inert gas to said enclosure to a pressure slightly less than atmospheric, heating the cup to above about 800 C. to melt the calcium and reduce the oxide to thorium powder, cooling and removing said cup from said enclosure, washing the admixed material from the thorium powder and drying said powder.

7. The method of manufacturing thorium comprising mixing the-oxide thereof withcalcium. placing the mixture in a, cup of high melting point material relatively inert to the charge and thorium at elevated temperatures, supporting said cup and placing it in a gas-tight enclosure to protect it and the thorium when produced from oxidation, exhausting said enclosure to a high degree of vacuum, introducing inert gas to said enclosure, heating the cup and contents to melt the calcium, start a reaction, and reduce the oxide to thorium powder, increasing the temperature of said cup to about 1400 C, to increase the fluidity of the charge and quickly effect a reaction more complete than usual, cooling and removing said cup from said enclosure, washing the admixed material from the thorium powder produced, and drying said powder. 'i

8. vThe method of manufacturing thorium comprising mixing the oxide thereof with calcium, placing the mixture in a cup of high-melting point metal relatively inert to the charge and thorium at elevated temperatures, supporting said cup and placing it in a gas-tight enclosure, exhausting said enclosure, introducing inert gas thereinto, heating the cup and contents to between about 800 and 900 C. to melt the calcium and cause it to react with the oxide and liberate free thorium, increasing the temperature of said cup to about 1400 C. to quickly effect a. reaction more complete than usual and holding it atsaid temperature for a moderate length of time, cooling and removing said cup from said enclosure, washing the admixed material from the thorium powder produced, and drying said powder.

9. The method of manufacturing thorium comprising mixing the oxide thereof with calcium, placing the mixture in a cup of high melting point material relatively inert to the charge and thorium' at elevated temperatures, supporting vsaid cup within a gas-tight enclosure, evacuating said enclosure, introducing inert gas thereinto, heating said cup to melt the calcium, starting the reaction, and reducing the oxide to thorium powder, cooling the mixture, washing the admixed `material from the thorium powder produced, and

drying said powder.

HARVEY C. RENTSCHLER. WILLIAM C. LILLIENDAHL. JOHN E. GRAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,088,909 Kuzel et al Mar. 3, 1914 1,568,685 Moore Jan. 5, 1926 1,573,083 Marden et al Feb, 16, 1926 1,704,257 Marden et al Mar.- 5, 1929 1,814,719 Marden et al. July 14, 1931 2,205,854 Kroll June 25, 1940 FOREIGN PATENTS Number Country Date 439,877 Germany Jan. 20, 1927 OTHER REFERENCES Kroll: zeitschrift fr Metankunde, Feb., 1936, pages 30-33. 

